1. Field of the Invention
This invention relates to optical time domain reflectometry (OTDR).
2. Related Art
In OTDR, an optical pulse is launched into an optical fibre (or more generally into a waveguide--in this specification the expression `optical fibre` is used to include the more general case of a waveguide as well as an optical fibre)--and backscattered signals returning to the launch end are monitored. In the event that there are discontinuities (such as faults or splices) in the fibre, the amount of backscattering generally increases, and any such change is detected in the monitored signals. Backscattering and reflection also occur from elements such as couplers, and so the monitored signals are usually compared with a reference record, new peaks and other changes in the monitored signal level or plot being indicative of changes in the fibre path, normally indicating a fault. The time between pulse launch and receipt of a backscattered signal is proportional to the distance along the fibre to the source of the backscattering, and so OTDR is a useful technique for fault location.
The rapid evolution of erbium-doped fibre amplifiers has freed telecommunications system designers from the constraints of opto-electronic regenerators. It is now possible to conceive of a future fixed transport layer where the links may be upgraded simply by modifying the terminal equipment. Similarly, it will reduce operations and maintenance costs if fault location can also be enabled remotely from the terminal locations. The application of OTDR to an optically amplified system was first successfully demonstrated for semiconductor laser amplifiers (SLAs)--see Blank & Cox--`Optical Time Domain Reflectometry on Optical Amplifier Systems`, Journal of Lightwave Technology, vol. 7, no. 10, pages 1549-1555. The major obstacle overcome was that the amplifiers produce amplified spontaneous emission (ASE), and this may be sufficient to overload the OTDR receiver. The technique used by Blank and Cox was primarily that of limiting the optical bandwidth of the test pulse and the ASE spectrum incident on the receiver, and using additional electrical processing in the OTDR to deal with the residual ASE. A narrowband optical filter was placed at the output of an OTDR instrument to limit the level of ASE entering the receiver. The optical output from the standard Fabry-Perot laser normally used in OTDR instruments would have been severely attenuated by the filter response, and so the OTDR laser was replaced with a narrow linewidth DFB laser, matched to the filter response. No further modifications were required to the commercial OTDR instrument (Hewlett-Packard HP8145A).
Unfortunately, this technique can only be used in systems which do not include in-line optical isolators. For the majority of systems, however, it is necessary (or at least desirable) to include at least one optical isolator in-line with each fibre amplifier to ensure system stability under all conditions. Thus, the control of ASE in systems having a plurality of amplifiers is important--if the ASE of one amplifier is sufficiently large, it will be amplified by the other amplifiers in the system (ASE travels in both directions), and this can cause the entire system to oscillate (lase). The use of in-line optical isolators prevents oscillations, as effectively light signals can travel in only one direction. Typical values for the isolation are likely to be in the range 30-40 dB. Not only does this preclude system upgrades to bi-directional working, but it also removes any possibilities for enabling fault location using OTDR at the system wavelength. A number of schemes have been suggested as ways of addressing this issue. Unfortunately, these schemes involve the use of optical circulators, routing the "go" and "return" paths through separate amplifiers, and the extra complexity involved is likely to be justified only in specialised applications.